Some vendors have started shipping DDR2-5300 memory that provides 667Mhz 4GB SODIMMs for you notebook. You will need a notebook that has a SantaRosa chipset to be able to access all 8GB and even then the BIOS has to support it.
I personally purchased 8GB G.Skill memory from newegg.com for $170 per chip. I have tested the memory in a Lenovo T61P and a Dell Latitude 830 with no issues. I did have an issue with a Dell Inspiron 1420. It does not recognize both 4GB chips when installed and does not finish the boot process. It does support a 4GB and a 2GB module for 6GB RAM.
So for about $350 you can upgrade your compatible notebook to 8GB RAM.
Hope this helps you run more VMs on your notebook. 🙂